The invention relates to a method for the formation of a layer and to an apparatus for its implementation. In particular, the invention relates to a method for the preparation of thin monocrystalline and thin polycrystalline layers on monocrystalline, polycrystalline, amorphous or other substrates, the substrate consisting of the same substance as the thin layer (epitaxy) or of a substance different from that of the thin layer.
The preparation of thin crystalline layers from various materials is of great importance for numerous applications. Actual examples of applications from the field of semiconductor technology are thin-layer solar cells. For these and numerous applications, the minority charge carrier lifetime should be long and, moreover, at least so long, that the free-path length of the minority carrier is large in comparison to the thickness of the crystalline layer. For this reason, layers with an overall lower defect density or lower density of charge carrier recombination centers are always superior for such applications to other layers, which have a higher density of recombination centers. Crystalline semiconductor layers, which are produced by liquid phase epitaxy from a solution, have a comparatively low density of defects and, in particular, of charge carrier recombination centers, assuming, of course, that they have been produced under suitable conditions.
Generally, it is necessary to resort to the use of high-grade and correspondingly expensive substrates for producing a layer of high quality, which is largely free of defects. On the other hand, there is increasing interest in the use of extremely inexpensive substrates, the crystallinity, volume properties and surface morphology of which are not perfect, as base materials for the thin layers. For the production of high grade crystalline layers on such base materials, one endeavors to keep the production temperature for the crystalline layers as low as possible. The advantages of a low production temperature are the reduction or avoidance of undesirable interactions with the base material, the resulting high quality of the crystalline layers, which manifests itself by the fact that the layers are largely free of recombination centers and have correspondingly low defect densities, as well as by saving energy. For thermodynamic reasons, the higher the growth temperature, the higher is the density of atomic defects. When growing crystals from solution, one generally works far below the melting temperature of the crystalline material. The high temperatures, which are required, for example, for the melting and subsequently recrystallizing of a material, are avoided.
However, when employing crystalline growth from solution, high supersaturation of the solution is required from time to time. When the whole volume of the solution is highly supersaturated, undesirable, homogeneous, spontaneous nucleation with subsequent parasitic growth of these nuclei can occur. The growth of these unwanted nuclei leads to a desaturation of the solution to the disadvantage of the intended crystallization process.
A further disadvantage of the state of the art lies therein that, in order to keep the solution constantly in a supersaturated state, a temperature difference must be maintained in the growing system.